Adult male Wistar rats were fed nutritionally adequate liquid diets providing 35% of the total calories as ethanol, while pair-fed controls received the corresponding diet with ethanol replaced by an equicaloric concentration of sucrose. After 2, 4, 6, 8, 10 and 14 weeks of chronic ethanol treatment, separate groups of rats were injected with a test dose of either ethanol (3 g/kg) or pentobarbital (40 mg/kg). Rectal temperatures were determined prior to and at 30, 60, 90 and 120 min after injection. The fall in rectal temperature after a test dose of ethanol or pentobarbital was significantly lower at 2 and 4 weeks in ethanol-treated rats than in pair-fed controls. After 6 and 8 weeks of chronic ethanol treatment, the fall in temperature after the same test doses was still less in ethanol-treated rats than in controls. However, at 10 and 14 weeks the fall in temperature was virtually identical in the two groups. A similar pattern of results was obtained when ethanol-induced sleep was compared in ethanol-treated rats and control rats. These intriguing results challenge the generally believed concept that tolerance and cross-tolerance, once produced, are sustained with chronic treatment, and may raise the possibility that such processes are reversible even during chronic treatment, at least under certain regimens.